1. Technical Field
The present invention generally relates to fluid flow, and more particularly to a variable orifice for metering and controlling fluid flow.
2. Related Art
In process control industries, it is common to use small diameter tubes to carry process fluids at low flow rates when small amounts of fluids are required for manufacturing processes. The tubes are almost always of a circular cross-section. Instruments used to measure a flow rate in the tubes must interface with a fluid flowing in the tube while minimizing disturbance to the fluid flow. To minimize disturbance to the fluid flow, the instrument typically includes a circular cross-section to match the cross-section of the tubes. The flow rate for a flow meter measuring a change in pressure across an orifice is defined by the following equation:
  Q  =      C    *          A      o        *                  (                  1                      1            -                                          (                                                      A                    o                                                        A                    P                                                  )                            2                                      )                    1        2              *                  (                  2          *                                    (                                                P                  hi                                -                                  P                  lo                                            )                        ρ                          )                    1        2            
Where:                Q=volumetric flow rate        C=orifice discharge coefficient        Ao=cross-sectional area of the orifice        Ap=cross-sectional area of the pipe        Phi=upstream pressure        Plo=downstream pressure        ρ=density of the fluid        
Flow meters used for measuring flow rates in small tubes may have the pressure sensors and orifice integrated in the same housing. Since the flow rate is a function of the cross-sectional area of the orifice, it is important to know with precision the size of the orifice opening. Typically, orifice based flow meters include an orifice having a fixed opening and the user is required to change the entire flow meter in order to obtain a different orifice size to accommodate different flow rates. Attempts have been made to produce flow meters with variable orifices. However, if the orifice opening does not retain a consistent shape as the size of the orifice opening is changed, errors result when calculating the flow rate using the above flow equation. For example, if a circular orifice compresses into a slightly elliptical shape rather than a perfect circular shape, an error may result when calculating the flow rate because the area value for the equation assumes the shape will remain circular. Also, the shape of the front edge and the rear edge of the orifice directly affect the discharge coefficient of the orifice and subsequent flow characteristics of the orifice. If the shape of the front edge or rear edge of the orifice changes with the size of the opening, flow characteristics of the orifice will change continuously. If the discharge coefficient is not consistent as the size of the orifice opening changes, and if it is not known with precision, errors will again result using the above flow equation.
Metering and controlling fluid flow is most commonly performed using separate devices or at least separate features included in a single device. For example, a device that meters fluid flow using an orifice may include a separate valve member that controls the amount and pressure of fluid flowing through the flow tubes, and therefore the orifice. In other applications, a separate valve device is positioned in the flow path before or after the metering device. In either scenario, the separate nature of the metering and controlling functions results in a bulky and often expensive arrangement for performing both metering and control of the fluid flow. Also, because the separate features must be connected together, additional seals or gaskets are required to prevent leaks.
Known variable orifice devices are ineffective for several reasons. First, known variable orifice devices typically use circular or curved members that are moved with respect to the fluid flow to change the size of the orifice. Because of the curved nature of these members, the shape of the orifice changes as the size of the orifice changes, which results in significant errors when calculating the fluid flow over a range of orifice sizes. Second, the changed shape of the orifice leads to non-ideal orifice shapes for at least a portion of the flow range. This leads to inconsistent flow characteristics for any given opening as flow rate changes, again leading to errors in the calculation of fluid flow.
A flow device that addresses these and other shortcomings of known flow control and metering devices would be an important advance in the art.